For short-run work, particularly for single copies or a few copies, incremental printers are far faster and more economical than printing presses. This enormous advantage flows from a totally different set of approaches, techniques and processes in the two technologies.
Incremental printers form colors through a set of essentially electromechanical procedures, though chemistry is important in the interaction between inks and printing media. These procedures are quite different from the fundamentally optical/photochemical (and modernly also computer-graphics) operations used in offset printing.
Incremental printing does have its own limitations and constraints. Such limitations can be appreciated from a comparison of the methods used for defining pixels, and forming colors, in the two very different technologies.
(a) Printing-press technology—Traditionally, pixels of offset negatives and plates are defined and shaded by extremely high-precision—and extremely expensive—camera lenses, factory preformed soft-dot screens, and exposure-time controls.
More modernly, as seen for instance in the patents of Dainippon Screen, offset pixels are in part defined and shaded by similarly high-precision, expensive electronic systems. In both of these conventional offset approaches, i. e. traditional and modern, the overriding technical and economic philosophy is to endow the professional printshop with fine precision make-ready apparatus that controls pixel geometry in a direct geometrical fashion.
The high cost of each make-ready apparatus can be amortized over make-ready work for many different presses, many millions of printing impressions, and many years of service. The equipment which thus defines where and how much ink will be placed is ordinarily an entirely different apparatus from the equipment which thereafter actually places the ink.
When one more printed copy of the image is desired, the operators need only rotate the printing drum once more—the colorant placement is already defined by the printing plate. Even if another copy is desired after the plate has been recycled (i. e. destroyed), the identical colorant placement can be reestablished from the printing negatives. There is no need to revert to the original information source, whether it was a visible camera-ready master or an electronic image-data computer file.
(b) Incremental printing technology—Incremental printing, by comparison, mechanically defines and simultaneously marks pixels by an ingenious complex of:    moving hardware,    orthogonally moving print medium,    split-millisecond timing,    an inexpensive consumable component known as a “printhead” or (in inkjet printing) “pen”, and    inks specially formulated to be amenable to ejection, flight and deposition without physical contact between hardware and medium.
In incremental printing each act, or operation, of mechanically defining a particular pixel thus serves—and instantaneously serves—one and only one application of colorant to the medium.
If another printed copy of the image is desired, the entire array of pixels must be mechanically redefined from scratch by the same hardware, the only commonality being the image-data computer file that defines informationally what the electromechanical hardware will later define mechanically. (As noted above, an information source is present in offset work too, but not normally consulted for each additional printout.)
Due to the extremely dynamic and transitory nature of this pixel-defining and -marking process—and particularly in view of the relatively humble and inexpensive printhead that is at the crux of this process—the resulting colorimetric tones are subject to significant variation. By the same token, however, the entire process at each point—being dynamic—is subject to pixel-wise control, and this point-by-point control is readily exploited to correct or compensate for undesired variation.
(c) Colorimetric nonlinearity—In particular, the variation just mentioned is often manifested in nonlinearity of tonal steps—in nominally linear calorimetric shadings. Linearity of tonal steps is extremely important to calorimetric accuracy.
Linearity is important not merely to the precise perceptible shade (e. g. lightness) of a single subtractive primary printed alone, but also to the hue and chroma of all complex shades formed by printing dots of the different primaries mixed together. Linearity in incremental printing requires, in effect, either:    (1) a very precise relationship between the size of each colorant dot and the size of each pixel that such a dot may occupy—or, alternatively,    (2) an arithmetic adjustment to each input tonal value to accommodate imprecision in that relationship.
When the dot-to-pixel size relationship is correct, then a nominally linear geometrical sequence of activated-pixel fractions produces a similarly linear sequence of actual inking fractions—without any need for arithmetic adjustment. Accomplishing linearity in this way, however, would be prohibitively expensive because nonlinearity can arise from minute tolerances in any of a great number of operating parameters.
These include for example the electronic timing of dot-formation commands, and interactions between the printing medium and the colorant; and, in inkjet work, distance of inkdrop flight from printhead to printing medium, nozzle size and directionality, heater chamber size, and heater firing energy. The latter four factors affect inkdrop volume, which in turn influences both dot size and dot placement. All the parameters mentioned also directly affect colorant dot coalescence with nearby dots.
(d) Linearization procedures—Hence in a practical, economical sense high-quality printing with incremental systems and methods requires actual measurement of tonal-step linearity, and retention of linearization correction coefficients or the like for use in printing images thereafter. This kind of correction is known in the art, and may be effectuated by any of various techniques—some open-loop, others closed-loop.
Some such procedures are centered upon factory measurements for each individual printer, or for an entire line of printers. Others are based on measurements made in the field (i. e. after distribution of the product), either automatically by programmed systems in each printer or by procedures prescribed for performance by human operators of the equipment—or partly automatic, partly manual.
All linearization procedures necessarily rely, at some point in the cumulative history of the overall data, whether in factory or field, upon printing and measurement of a test pattern. Such measurement is followed by feed-back of measured errors as correction signals to color-adjusting stages in the printing system of the individual printer.
It is well known in this field that results of linearization are different when a printing system is using different inks, or different printing media—or both. Therefore, the system linearization procedure must be repeated whenever a new set of printheads (pens, in inkjet systems) is placed into service—and also whenever the print media are changed.
The printing and measurement procedures that are needed to accomplish the linearization, described above, necessarily consume both ink and printing medium—as well as time. Since linearization is fundamental to good image quality, however, these investments of resources are well spent.
(e) Linearization hardware—In the measurement phases of linearization, a reliable measuring device is required. This may be a high-quality calorimeter—for instance a free-standing one such as mentioned in the coowned U.S. Pat. No. 5,272,518 of Vincent, or a printer-mounted one such as taught in another coowned patent of Vincent, U.S. Pat. No. 5,671,059, or in the above-mentioned patent document of Thomas Baker.
A calorimeter is or can be made direct-reading in perceptual colorimetric space, such as the well-known CIELAB space. Such direct perceptual readout is very favorable, since it is in perceptual terms that a printing system ideally should be linearized.
Alternatively and much more economically, however, the measuring device can be a simple densitometer, or even a relatively crude optical sensor that is custom driven—and whose output signals are specially interpreted—to yield values that the Baker document terms “pseudodensito-metric” measurements. Such a device is especially favorable in a production-printer environment, for measurements to be made in the field after product distribution, because many or most sophisticated incremental printers already include such a sensor for other uses.
In particular a simple optical sensor—often denominated a “line sensor”—is provided for such purposes as pen alignment, and other strictly positional calibrations. (A representative application of such a sensor is taught in the Sievert patent document mentioned earlier.) In scanning printers, a line sensor ordinarily is mounted to the carriage that holds the printheads and scans them back and forth across the printing medium.
Actually for such usages a sensor need do little more than distinguish dark from light. This is accordingly the type of sensor that a calorimetric calibration module can, in effect, inherit from the general operations of an incremental printer.
The line sensor consists of a light source and an electrooptical detector. The source illuminates the print medium and whatever marks have been printed upon it, and the detector produces an electrical signal related to the light reflected from the medium and those marks. In practice the source is often a light-emitting diode, or in better units two such diodes emitting light of different colors so that the sensor can respond suitably to the several subtractive primary colorants used in printing.
(f) Fine linearization with modest equipment—The challenge then becomes how to infuse such a primitive device with an adequately close approximation to the high-quality measuring capabilities of a perceptual-reading calorimeter—or, more precisely, how to do so at minimal cost and complexity. It is conventional in the art of incremental printing to meet this challenge by calibrating the sensor itself, in perceptual terms, and storing the calibration data for use whenever a linearization is to be performed.
There have been several different overall approaches to providing such a calibration of the sensor. The calibration, at least in principle, can be performed either at the factory or in the field—but factory calibration is the only prior method which the present inventors know was actually commercialized:
If calibration is performed at the factory, the data for all media then known are stored in the printer memory and must be invoked—by media type—before linearization for any particular combination of media and inks or pens. Factory calibration is undesirable for several reasons that will be taken up shortly.
Commercialization of factory calibration for a line sensor has occurred in a Hewlett Packard printer/plotter. In that product the calibration was performed for a statistically significant number of different sensor units in production printers, and—from the resulting data—weighted-mean calibration values were adopted for the product line as a whole.
In one variant of this sensor-calibration strategy, different data sets have been acquired for different basic types of ink: pigment inks and dye inks respectively. In this case, different calibration values have still been adopted for the product line as a whole—but these values have been tabulated separately for the different ink types.
In another variant of the factory sensor-calibration strategy, different data sets have been acquired for different subpopulations of sensors. Such subpopulations arose from tolerances in the light sources, detectors and geometries of the sensors and diverged significantly in the conversion factors that they generated through the calibration process.
Accordingly in this case different calibration values were adopted for each subpopulation of sensors, rather than for the product line as a whole—but nevertheless as a matter of convenience, if preferred, all the different subpopulation sets could be stored in all the printers. This was particularly useful in case the sensor had to be changed after a printer has been distributed and is in the field, i. e. in an end-user's facility.
In any event, the calibration values were then saved in the printer memory for all the machines in the product line, and/or in some part of the product line carrying a particular respective sensor subpopulation. In all cases, separate calibration numbers were saved for each different printing medium.
As indicated above, although field calibration of the sensor has been possible in principle, the present inventors are not aware of any prior commercialization of that approach. If sensor calibration is performed in the field, then presumably it is done whenever a new set of colorants or printheads (or both) is placed in service—and also whenever a different type of printing medium is first placed in service.
Since we assume here that the printer is available for sensor calibration, one field-calibration strategy is to conserve printer memory space by calling for the calibration to be performed shortly before each linearization; then only one set of media data need be stored at any one time.
An alternative strategy is to simplify the operation or usage of the machine by storing many sets of media data—upon acquiring such data in the field—and then calling up a suitable set by media type, as in the factory sensor-calibration case.
In any event, after sensor calibration, as noted above, the system is ready to perform a linearization for the inks and medium then in use. It has been natural to perform such changes in calibration, like the linearization, for each different printing medium because in the linearization process—as also mentioned above—the sensor responded differently to test patterns printed on different media. In other words, the requirement to calibrate the sensor separately for each different printing medium was grounded in the requirement to linearize separately for each change of print medium.
(g) Drawbacks in conventional calibration—Unfortunately, regardless of which of the above-discussed approaches and strategies is adopted, several problems result:
First, when sensor calibration is performed in the factory any media introduced by the printer manufacturer after a particular printer has been distributed to an end-user—and third-party media as well—are absent from the media-data memory. This is a serious problem because special provision must then be made for use of such media, or the media are usable only without proper linearization.
Another serious problem is that calibration of a line sensor is not strictly accurate unless it is performed using the particular ink sets and printheads that will be used in the linearization and subsequent printing.
Yet another problem is that memory storage space in the printer must be dedicated to the calibration data. The amount of data, however, is modest and this problem is secondary.
Second, if sensor calibration is performed in the field (if in fact this has been done), while this mitigates the problem of third-party and postintroduction media—and as well the problem of inaccuracy due to calibration without actual inks and pens to be used—it does introduce other difficulties. Adoption of the first-mentioned field-calibration strategy (calibrating a sensor separately for each linearization) raises the end-user's inconvenience and expense:
Recalibration of the sensor after each media change (in addition to relinearization of the printing system after each media change, and also in addition to recalibration of the sensor after each ink-set or pen change) makes the overall process very time consuming, and somewhat expensive in materials cost as well.
This first field-calibration strategy generally doubles the time required for linearization alone. The dual process also consumes quantities of ink and printing medium, again roughly doubling the quantities expended for linearization only.
If instead the second-mentioned field-calibration strategy (of storing all the different media-data sets) is adopted, then the problems just discussed are somewhat reduced but still important if the user wishes to use several different media—particularly trying or switching to new media as they become available. Furthermore the objectionable cost of dedicated memory space arises again, as in the factory-calibration case.
Thus from the standpoint of a user of the system, a separate sensor calibration for each different type of medium—preliminary to relinearization—is extremely undesirable. What is desired is some more-efficient way to prepare for the necessary linearization.
(h) Dynamic-range adjustment—For purposes of this document it is important to distinguish another type of conventional sensor setting that is sometimes (though not in this document) called “sensor calibration”, but is a much lower-level matter and strictly electronic rather than related to colorimetry (or even pseudodensitometry) as such. This is an automatic, routine and rapid procedure that is performed both in the field—i. e. in the facilities of an end-user—and in the factory.
It is performed before—usually just before—operation of each algorithm that will use the line sensor, in particular just before a linearization or a sensor calibration. Its purpose is basically to assure that there is enough optical signal on the sensor, and thereby enough electrical signal from the sensor.
This procedure prints a color patch on white paper for each ink color and calculates optimum electronics settings (gain and offset on the signal amplifiers) for each color on the given printing medium, and for the bare medium; however, as suggested above, this calibration has no direct relationship with color. For each patch and particularly for the bare medium, the LED current is raised until adequate signal appears—most typically eight or sixteen steps on the analog-to-digital converter (“ADC”)—and then until the sensor signal saturates; and then the current is lowered slightly to establish suitable dynamic range.
The purpose of this procedure is only to optimize the ADC dynamic range, and improve the electronic signal-to-noise ratio, for a given printing medium and ink set. For purposes of this document, this procedure is not a “calibration” and will be called a “dynamic-range adjustment”.
(i) Conclusion—As this discussion suggests, limitations in efficiency of preparing an incremental printer system for use continue to impede achievement of uniformly excellent inkjet printing that is within the constraints of acceptability to product buyers, owners and operators. Thus important aspects of the technology used in the field of the invention are amenable to useful refinement.